Systems and methods of optical path protection for distributed antenna systems

BACKGROUND

Fiber breaks and/or degradation in a fiber feeding a distributed antenna system (DAS) network can have a large impact on a system's availability. Network operators typically look for a high-degree of network availability (for example, “0.9999” or higher). A network availability of 0.9999 requires a network be unavailable for no more than 1 hour per year, whereas a network availability of 0.99999 requires a network be unavailable for no more than 1 hour every ten years. One fiber break that causes a DAS network to be unavailable for 10 hours while the fiber break is isolated and repaired would lower availability to 0.999. Oftentimes fiber breaks take even longer to isolate and repair.

Adverse changes to a fiber-based network can happen in many different ways. Examples include digging near fiber optic cabling that causes damage to fiber, damaging the cable by a maintenance worker stepping on, bending, or breaking a fiber. Aerial-mounted fiber can be damaged during accidents involving the poles hanging the fiber. Optical connectors can become degraded by damage and/or dirt. These are just a few of the fiber degradation scenarios, any of which can cause degraded or loss of service on that fiber for extended periods of time. Due to the time required to isolate and repair fibers, redundant paths are often used in order to keep the system up as much as possible. Since fiber damage tends to be a local event, redundant fibers are usually run via completely different paths. In this way, any local damage done to the fiber does not affect the redundant path. In one application of a DAS network that makes use of redundant fibers, both a hub and remote node connected by the redundant fibers includes active switching devices in order to switch from a primary fiber to a secondary fiber in the event of a break (or other degradation of the primary fiber). However, remote nodes are often located in isolated areas with limited power resources and space for accommodating optical switching equipment.

SUMMARY

In one embodiment, a system is provided. The system comprises a hub; at least one remote node that is located remotely from the hub; wherein the hub communicates with the at least one remote node via either one of a primary fiber path and a backup fiber path, the primary fiber path comprising an uplink fiber and a downlink fiber and the backup fiber path comprising an uplink fiber and a downlink fiber; wherein the at least one remote node is coupled to the downlink fiber of the primary fiber path and the downlink fiber of the backup fiber path via an optical combiner; wherein the at least one remote node is further coupled to the uplink fiber of the primary fiber path and the uplink fiber of the backup fiber path via an optical splitter; and wherein the hub switches from communicating with the at least one remote node on the primary fiber path to communicating with the at least one remote node on the backup fiber path based on an uplink optical signal received from the at least one remote node.

In another embodiment, a method is provided. The method comprises receiving an electrical uplink radio frequency signal; generating an uplink optical signal derived from the electrical uplink radio frequency signal; splitting the uplink optical signal for transmission on a primary uplink optical fiber and a secondary uplink optical fiber; combining any downlink optical signal received on a primary downlink optical communication medium and any downlink optical signal received on a second downlink optical communication medium in order to output a downlink optical signal; and generating a downlink radio frequency signal derived from the downlink optical signal.

In another embodiment, an apparatus is provided. The apparatus comprises means for receiving an electrical uplink radio frequency signal; means for generating an uplink optical signal derived from the electrical uplink radio frequency signal; means for splitting the uplink optical signal for transmission on a primary uplink optical fiber and a secondary uplink optical fiber; means for combining any downlink optical signal received on the primary downlink optical communication medium and any downlink optical signal received on the second downlink optical communication medium in order to output a downlink optical signal; and means for generating a downlink radio frequency signal derived from the downlink optical signal.

BRIEF DESCRIPTION OF THE DRAWINGS

<figref idrefs="DRAWINGS">FIG. 1</figref> is a block diagram of one embodiment of a distributed antenna system.

<figref idrefs="DRAWINGS">FIG. 2</figref> is a block diagram of one embodiment of a distributed antenna system.

<figref idrefs="DRAWINGS">FIG. 3A</figref> is a block diagram of one embodiment of a distributed antenna system.

<figref idrefs="DRAWINGS">FIG. 3B</figref> is a block diagram of one embodiment of a distributed antenna system.

<figref idrefs="DRAWINGS">FIG. 4</figref> is a flow chart of a one embodiment of a method of providing passive optical path protection.

<figref idrefs="DRAWINGS">FIG. 5</figref> is a block diagram of one embodiment of a distributed antenna system.

DETAILED DESCRIPTION

<figref idrefs="DRAWINGS">FIG. 1</figref> is a block diagram of one embodiment of a distributed antenna system (DAS) <b>100</b>. DAS <b>100</b> is often used in a wireless communication network (for example, a cellular wireless network) to communicatively couple one or more base stations (not shown in <figref idrefs="DRAWINGS">FIG. 1</figref>) to one or more antennas that are remotely located from the base stations (not shown in <figref idrefs="DRAWINGS">FIG. 1</figref>). DAS <b>100</b> includes a hub <b>110</b> located near the base stations and one or more remote access nodes <b>130</b> (also referred to here as “remote nodes” <b>130</b>), each of which is located near one or more remote antennas (for example, a primary and diversity antenna). For the purposes of illustration, the DAS <b>100</b> comprises a single hub <b>110</b> that is optically coupled to single remote node <b>130</b> (though it is to be understood that in other embodiments other numbers of hubs <b>110</b> and/or remote nodes <b>130</b> are used). The hub <b>110</b> is optically coupled to the remote node <b>130</b> through a primary optical fiber path <b>120</b> and a backup optical fiber path <b>125</b>. In the embodiment illustrated in <figref idrefs="DRAWINGS">FIG. 1</figref>, primary optical fiber path <b>120</b> includes a primary downlink optical fiber <b>122</b> and a primary uplink optical fiber <b>124</b>. Backup optical fiber path <b>125</b> includes a secondary downlink optical fiber <b>127</b> and a secondary uplink optical fiber <b>129</b>. In alternate embodiments, one or both of primary optical fiber path <b>120</b> and backup optical fiber path <b>125</b> include a plurality of uplink and/or downlink optical fibers.

Hub <b>110</b> comprises interface functionality <b>111</b> that couple the hub <b>110</b> to one or more base stations, a fiber optic transceiver <b>112</b> communicatively coupled to the one or more interface functionality <b>111</b>, an optical switch <b>114</b> communicatively coupled to the fiber optic transceiver <b>112</b>, primary optical fiber path <b>120</b> and backup optical fiber path <b>125</b>, and a hub optical path protection (OPP) manager <b>116</b> communicatively coupled to the fiber optic transceiver <b>112</b> and the optical switch <b>114</b>. In the downlink direction, the interface functionality <b>111</b> of the hub <b>110</b> receives analog downlink RF signals from the one or more base stations to which the hub <b>110</b> is communicatively coupled and provide the fiber optical transceiver <b>112</b> a suitable electrical signal for modulating onto a downlink optical carrier. The downlink optical signal output by the fiber optic transceiver <b>112</b> is selectively communicated to the remote node <b>130</b> on either the primary downlink optical fiber <b>122</b> or secondary downlink optical fiber <b>127</b> by the optical switch <b>114</b>, depending on a fiber path control signal output by the hub optical path protection manager <b>116</b>.

In one implementation of such an embodiment (also referred to here as a “digital-transport implementation”), the interface functionality <b>111</b> receives one or more analog downlink radio frequency (RF) signals from each of the base stations to which the hub <b>10</b> is communicatively coupled and digitizes at least a portion of the received analog downlink RF signal (for example, by digitizing a particular frequency band of the received analog downlink RF signal). Also, in such a digital-transport implementation, the interface functionality <b>111</b> combines at least a portion of the digitized downlink RF signals from one or more base stations into frames suitable for transmission on the primary downlink optical fiber <b>122</b> and/or the secondary downlink optical fiber <b>127</b> (for example, by formatting the at least a portion of the digitized downlink RF signals into SONET STS-48/OC-48 formatted frames). In such a digital-transport implementation, the fiber optic transceiver <b>112</b> digitally modulates the electrical signal (which comprises frames of digitized downlink RF signals) onto a downlink optical carrier in order to generate the downlink optical signal. An example of downlink functionality suitable for use in such a digital-transport implementation is described in U.S. Pat. No. 6,963,552, titled “MULTI-PROTOCOL DISTRIBUTED WIRELESS SYSTEM ARCHITECTURE” (also referred to here as the “'552 Patent”), which is hereby incorporated herein by reference.

In an alternative implementation of such an embodiment (also referred to here as an “analog-transport implementation”), the interface functionality <b>111</b> receives one or more analog downlink RF signals from each of the base stations to which the hub <b>110</b> is communicatively coupled and filters, combines, mixes, and/or splits the received analog downlink RF signals into a single electrical analog signal suitable for transmission on the primary downlink optical fiber <b>122</b> and/or the secondary downlink optical fiber <b>127</b>. In such an analog-transport implementation, the fiber optic transceiver <b>112</b> amplitude modulates the single electrical analog signal received from the interface functionality <b>111</b> onto a downlink optical carrier in order to generate the downlink optical signal.

In the uplink direction, hub <b>110</b> receives an uplink optical signal from the remote node <b>130</b> on both of the primary uplink optical fiber <b>124</b> and the secondary uplink optical fiber <b>129</b>. Optical switch <b>114</b> selectively couples one of the primary uplink optical fiber <b>124</b> and the secondary uplink optical fiber <b>129</b> to the fiber optic transceiver <b>112</b>, depending on the fiber path control signal from the hub optical path protection manager <b>116</b>. That is, when optical switch <b>114</b> selectively couples the primary uplink optical fiber <b>124</b> to the fiber optical transceiver <b>112</b>, any uplink optical signal received on the primary uplink optical fiber <b>124</b> is communicated to the fiber optic transceiver <b>112</b> by the optical switch. Likewise, when optical switch <b>114</b> selectively couples the secondary uplink optical fiber <b>129</b> to the fiber optical transceiver <b>112</b>, any uplink optical signal received on the secondary uplink optical fiber <b>129</b> is communicated to the fiber optic transceiver <b>112</b> by the optical switch. The fiber optic transceiver <b>112</b> demodulates the uplink optical signal in order to extract an electrical uplink RF signal, which is provided to one or more base stations via the interface functionality <b>111</b>. In the digital-transport implementation of such an embodiment noted above, the extracted uplink RF signal comprises frames (for example, SONET STS-48/OC-48 formatted frames) containing digitized uplink RF signals, which the interface functionality <b>111</b> extracts from the frames and converts to analog uplink RF signals. The analog uplink RF signals, in such an embodiment, are provided to one or more base stations coupled to the hub <b>110</b>. In the analog-transport implementation of such an embodiment noted above, the extracted uplink RF signal comprises an analog uplink RF signals that are provided to one or more base stations coupled to the hub <b>110</b>.

Remote node <b>130</b> comprises an optical combiner <b>134</b>, an optical splitter <b>135</b>, a fiber optic transceiver <b>132</b>, interface functionality <b>133</b>, and a remote optical path protection (OPP) manager <b>136</b>.

The optical combiner <b>134</b> of the remote node <b>130</b> receives downlink optical signals from hub <b>110</b> via one of either primary downlink optical fiber <b>122</b> or secondary downlink optical fiber <b>127</b>. An optical “combiner”, as the term is used in this specification, means a device that receives a plurality of optical signal inputs and combines the optical signals into a single optical output. The downlink optical signal is transmitted on only one of the primary downlink optical fiber <b>122</b> or the secondary downlink optical fiber <b>127</b> at any one time. For this reason, at any one time, only the selected one of the primary downlink optical fiber <b>122</b> or the secondary downlink optical fiber <b>127</b> is “lit” while the fiber not selected by optical switch <b>114</b> is “dark” and does not carry an optical signal. This eliminates concerns of optical interference at the optical combiner <b>134</b>. The resulting output of optical combiner <b>134</b>, which is essentially the combination of the downlink optical signal from the lit fiber with a null signal from the dark fiber, is thus simply the downlink optical signal. The output of the optical combiner <b>134</b> (that is, the downlink optical signal) is communicated via a single fiber to fiber optic transceiver <b>132</b>. The fiber optic transceiver <b>132</b> demodulates the downlink optical signal in order to extract an electrical downlink RF signal, which is provided to one or more antennas <b>140</b> and <b>141</b> via the interface functionality <b>133</b>. In the digital-transport implementation of such an embodiment noted above, the electrical downlink RF signal output by the fiber optic transceiver <b>132</b> comprises digitized downlink RF signals from one or more base stations. In such an implementation, the interface functionality <b>133</b> converts the digitized downlink RF signals into analog downlink RF signals (for example, using a suitable digital-to-analog conversion process), which are amplified for radiation from the one or more antennas <b>140</b> and <b>141</b>. In one example, the digitized downlink RF signals from each of the base stations is separately converted into an individual analog downlink RF signal for that respective base station (for example, by respective “RAN slices” of the type described in the '552 Patent) and each of the individual analog downlink RF signals are thereafter combined for amplification and radiation from one or more of the antennas <b>140</b> and <b>141</b>. In the analog-transport implementation of such an embodiment noted above, the electrical downlink RF signal output by the fiber optic transceiver <b>132</b> comprises analog downlink RF signals from one or more of base stations and the interface functionality <b>133</b> amplifies for radiation from the one or more antennas <b>140</b> and <b>141</b>.

In the uplink direction, the interface functionality <b>133</b> of the remote node <b>130</b> receives analog uplink RF signals from the one or more antennas <b>140</b> and <b>141</b> coupled to the remote node <b>130</b>. The interface functionality <b>133</b> provides the fiber optical transceiver <b>132</b> a suitable electrical signal for modulating onto an uplink optical carrier. The uplink optical signal output by the fiber optic transceiver <b>132</b> is communicated on both of primary uplink optical fiber <b>124</b> and secondary uplink optical fiber <b>129</b> by the optical splitter <b>135</b>. An optical “splitter”, as the term is used in this specification, means a device that replicates an optical signal received at an input to each of a plurality of outputs. That is, optical splitter <b>135</b> optically replicates the uplink optical signal output by the fiber optic transceiver <b>132</b> so that the uplink optical signal is communicated to hub <b>110</b> via both of the primary uplink optical fiber <b>124</b> and the secondary uplink optical fiber <b>129</b>. In the digital-transport implementation of the embodiment shown in <figref idrefs="DRAWINGS">FIG. 1</figref> noted above, the interface functionality <b>133</b> of the remote node <b>130</b> receives one or more analog uplink RF signals from each of the antennas <b>140</b> and <b>141</b> to which the remote node <b>130</b> is communicatively coupled and digitizes at least a portion of the received analog uplink RF signal (for example, by digitizing a particular frequency band of each analog uplink RF signal). Also, in such a digital-transport implementation, the interface functionality <b>133</b> combines at least a portion of the digitized uplink RF signals from one or more of the antennas <b>140</b> and <b>141</b> into frames suitable for transmission on the primary uplink optical fiber <b>124</b> and the secondary uplink optical fiber <b>129</b> (for example, by formatting the at least a portion of the digitized uplink RF signals into SONET STS-48/OC-48 formatted frames). In such a digital-transport implementation, the fiber optic transceiver <b>132</b> digitally modulates the electrical signal (which comprises frames of digitized uplink RF signals) onto an uplink optical carrier in order to generate the uplink optical signal.

Alternatively, in the analog-transport implementation of the embodiment shown in <figref idrefs="DRAWINGS">FIG. 1</figref> noted above, the interface functionality <b>133</b> receives one or more analog uplink RF signals from each of the antennas <b>140</b> and <b>141</b> to which the remote node <b>130</b> is communicatively coupled and filters, combines, mixes, and/or splits the received analog uplink RF signals into a single analog signal suitable for transmission on the primary uplink optical fiber <b>124</b> and the secondary uplink optical fiber <b>129</b>. In such an analog-transport implementation, the fiber optic transceiver <b>132</b> amplitude modulates the single electrical analog signal received from the interface functionality <b>133</b> onto an uplink optical carrier in order to generate the uplink optical signal, which is then communicated to the hub <b>110</b> on both the primary uplink optical fiber <b>124</b> and the secondary uplink optical fiber <b>129</b> via the splitter <b>135</b>.

Under normal operating conditions, communications between hub <b>110</b> and remote node <b>130</b> are conducted over the primary optical fiber path <b>120</b>. Normal operating conditions exist when communications between hub <b>110</b> and remote node <b>130</b> using the primary downlink optical fiber <b>122</b> and the primary uplink optical fiber <b>124</b> are accomplished within acceptable operating parameters discussed below.

The hub OPP manager (HOM) <b>116</b> determines when communications between hub <b>110</b> and remote node <b>130</b> via the primary fiber path <b>120</b> are within acceptable operating parameters. When HOM <b>116</b> determines that primary fiber path <b>120</b> is operating within acceptable operating parameters HOM <b>116</b> causes optical switch <b>114</b> to align itself for transmitting the downlink optical signal via the primary downlink optical fiber <b>122</b> and receiving the uplink optical signal from the primary uplink optical fiber <b>124</b> (the “normal state” of optical switch <b>114</b>). When HOM <b>116</b> determines that communications via primary fiber path <b>120</b> are not within acceptable operating parameters, HOM <b>116</b> causes optical switch <b>114</b> to align itself for transmitting the downlink optical signal via the secondary downlink optical fiber <b>127</b> and to align itself for receiving the uplink optical signal from the secondary uplink optical fiber <b>129</b> (the “backup state” of optical switch <b>114</b>).

HOM <b>116</b> determines when communications via primary fiber path <b>120</b> are not within acceptable operating parameters based on the uplink optical signal received on the primary uplink optical fiber <b>124</b>.

In one implementation of DAS <b>100</b>, fiber optic transceiver <b>112</b> communicates an uplink signal quality signal to HOM <b>116</b> (or other information indicative of an attribute of the uplink optical signal). When the uplink signal quality of the uplink optical signal degrades below a predetermined threshold level, HOM <b>116</b> switches optical switch <b>114</b> from its normal state to the backup state in order to utilize backup fiber path <b>125</b>. In one implementation, fiber optic transceiver <b>112</b> communicates the bit error ratio (BER) of the uplink optical signal it receives. In such an implementation, when the BER drops below a predetermined threshold BER, HOM <b>116</b> switches optical switch <b>114</b> from its normal state to the backup state in order to utilize backup fiber path <b>125</b>. As would be appreciated by one skilled in the art upon reading this specification, in a digital-transport implementation of a DAS <b>100</b>, a typical good fiber path performs at better than a 10e-12 BER. Because of the amount of over-sampling typically used in such a digital-transport implementation, an optical fiber path can continue to operate down to as low as 10e-6 BER without adversely affecting call quality. In other implementations, other signal quality indicators, such as but not limited to optical power levels, are used to determine whether or not communications via primary fiber path <b>120</b> are within acceptable operating parameters. For example, in another implementation, fiber optic transceiver <b>112</b> measures and communicates the optical power level of the uplink optical signal it receives. In such an implementation, when the optical power level drops below a predetermined threshold power level, HOM <b>116</b> switches optical switch <b>114</b> from its normal state to the backup state in order to utilize backup fiber path <b>125</b>.

Because hub <b>110</b> receives only the uplink optical signals, HOM <b>116</b> cannot directly determine the signal quality of downlink optical signals received by remote node <b>130</b>. Therefore, information regarding the signal quality of downlink optical signals is communicated to HOM <b>116</b> by remote OPP manager (ROM) <b>136</b>. In other embodiments, other information indicative of an attribute of the downlink optical signal is determined and communicated.

In one implementation, fiber optic transceiver <b>132</b> receives the downlink optical signal and communicates a downlink signal quality signal to ROM <b>136</b>. In one implementation, fiber optic transceiver <b>132</b> communicates the bit error rate (BER) of the downlink optical signal it receives to ROM <b>136</b>. In such an implementation, when the BER drops below a predetermined threshold level, ROM <b>136</b> reports the BER to HOM <b>116</b> so that HOM <b>116</b> can make the determination on whether to switch from the primary fiber path <b>120</b> to the backup fiber path <b>125</b>. In other implementations, other signal quality indicators are used to determine whether or not communications via primary fiber path <b>120</b> are within acceptable operating parameters. When the downlink signal quality reported by ROM <b>136</b> degrades below a predetermined threshold level, HOM <b>116</b> switches optical switch <b>114</b> from its normal state to the backup state in order to utilize backup fiber path <b>125</b>. For example, in another implementation, fiber optic transceiver <b>132</b> measures and communicates the optical power level of the downlink optical signal it receives. In such an implementation, when the optical power level drops below a predetermined threshold power level, ROM <b>136</b> reports the optical power level to HOM <b>116</b> so that HOM <b>116</b> can make the determination on whether to switch from the primary fiber path <b>120</b> to the backup fiber path <b>125</b>.

As would be appreciated by one skilled in the art upon reading this specification, it is not necessary for uplink and downlink signal quality to be based on the same criteria. For example, in one implementation fiber optic transceiver <b>112</b> measures and communicates the optical power level of the uplink optical signal it receives while fiber optic transceiver <b>132</b> communicates the bit error rate (BER) of the downlink optical signal it receives. In another implementation fiber optic transceiver <b>112</b> communicates the bit error rate (BER) of the uplink optical signal it receives while fiber optic transceiver <b>132</b> communicates the optical power level of the downlink optical signal it receives.

In another implementation, ROM <b>136</b> makes the determination whether to switch communications from the primary fiber path <b>120</b> to the backup fiber path <b>125</b> based on the signal quality of the downlink optical signal, and alerts HOM <b>116</b> of a degraded downlink fiber path by shutting off the uplink fiber path. In one such implementation, when the BER of the downlink optical signal drops below the predetermined threshold level, ROM <b>136</b> disables fiber optic transceiver <b>132</b> from transmitting the uplink optical signal to hub <b>110</b>. In another such implementation, when the optical power level of the downlink optical signal drops below the predetermined threshold level, ROM <b>136</b> disables fiber optic transceiver <b>132</b> from transmitting the uplink optical signal to hub <b>110</b>. Fiber optic transceiver <b>112</b> detects this loss of the uplink optical signal and outputs an uplink signal quality signal that indicates the loss of the uplink optical signal. HOM <b>116</b> responds to what it perceives as a degraded uplink signal quality by switching optical switch <b>114</b> from its normal state to the backup state in order to utilize backup fiber path <b>125</b>.

In the embodiment shown in <figref idrefs="DRAWINGS">FIG. 1</figref>, all the fiber switching is performed at the hub <b>110</b> rather than at remote node <b>130</b>. The hub <b>110</b> contains all of the “active” optical path protection circuitry (that is, the optical switch <b>114</b>) that is used to switch among the redundant fiber paths. The remote node <b>130</b> includes passive optical splitters and combiners that typically do not require significant space to install (space often being a scarce resource at remote node locations) and typically do not require addition power to operate (power often being a scarce resource at remote node locations). Further, when space is very limited within a remote node, the optical splitter and optical combiner need not be located within the remote node housing, but can be installed external to the remote node housing, as illustrated in the embodiment shown in <figref idrefs="DRAWINGS">FIG. 2</figref>. <figref idrefs="DRAWINGS">FIG. 2</figref> illustrates a hub <b>210</b> coupled to an optical combiner <b>234</b> and an optical splitter <b>235</b> via a primary optical fiber path <b>220</b> and a backup fiber path <b>225</b>. Hub <b>210</b> operates as described with respect to hub <b>110</b> shown in <figref idrefs="DRAWINGS">FIG. 1</figref>, transmitting downlink optical signals to optical combiner <b>234</b> and receiving uplink optical signals from optical splitter <b>235</b>. As illustrated in <figref idrefs="DRAWINGS">FIG. 2</figref>, optical combiner <b>234</b> and optical splitter <b>235</b> are located in a housing <b>215</b> that is external to remote node <b>230</b>. Optical combiner <b>234</b> and optical splitter <b>235</b> are coupled to the fiber optic transceiver <b>232</b> within remote node <b>230</b> via fiber pair <b>220</b>. Because optical combiner <b>234</b> and optical splitter <b>235</b> do not require power to operate, housing <b>215</b> may comprise an underground cable vault or similar location where power is not available. Further, only a single fiber pair <b>220</b> needs to be run between housing <b>215</b> and the remote node <b>230</b>, thus reducing system installation costs. A ROM <b>236</b>, interface functionality <b>233</b>, and fiber optic transceiver <b>232</b> within remote node <b>230</b> otherwise operate as described with respect to ROM <b>136</b>, interface functionality <b>133</b>, and fiber optic transceiver <b>132</b> in <figref idrefs="DRAWINGS">FIG. 1</figref>. The approach shown in <figref idrefs="DRAWINGS">FIG. 2</figref> is also suitable for use in “retro-fitting” or “aftermarket” applications where a service provider desires to add optical path protection for deployed remote nodes that are not designed to house an optical combiner or an optical splitter. In such an application, the functionality of the ROM is implemented in an element management system that is otherwise coupled to the remote nodes, and the protection switching performed by the optical switch in the respective hub is controlled by communications from the element management system that is otherwise communicatively coupled to the hub.

Although the embodiment shown in <figref idrefs="DRAWINGS">FIG. 1</figref> is described, for the purposes of illustration, as using a single optical wavelength on each fiber, it is to be understood that in other embodiments multiple optical wavelengths can be used. One such embodiment is illustrated in <figref idrefs="DRAWINGS">FIG. 3A</figref>. In the embodiment of <figref idrefs="DRAWINGS">FIG. 3A</figref>, hub <b>310</b> communicates with a plurality of remote nodes <b>330</b>-<b>1</b> to <b>330</b>-N via multiple wavelength optical signals. Hub <b>310</b> includes the functionality required to multiplex multiple optical carrier signals from a communications network (not shown) onto a single optical fiber by using different wavelengths of light to carry a plurality of different RF signals. Hub <b>310</b> comprises interface functionality <b>311</b>-<b>1</b> to <b>311</b>-x that couple the hub <b>310</b> to one or more base stations (not shown). The hub <b>310</b> further comprises fiber optic transceivers <b>312</b>-<b>1</b> to <b>312</b>-x communicatively coupled, respectively, to the interface functionality <b>311</b>-<b>1</b> to <b>311</b>-x, and wave division multiplexing (WDM) multiplexer (MUX) <b>313</b> communicatively coupled to the downlink output of fiber optical transceivers <b>312</b>-<b>1</b> to <b>312</b>-x. The hub <b>310</b> further comprises a WDM DE-MUX <b>315</b> communicatively coupled to the uplink inputs of optical transceivers <b>312</b>-<b>1</b> to <b>312</b>-x and an optical switch <b>314</b> communicatively coupled to WDM MUX <b>313</b>, WDM DE-MUX <b>315</b>, primary optical fiber path <b>320</b> and backup optical fiber path <b>325</b>. The hub <b>310</b> further comprises a hub optical path protection manager (HOM) <b>316</b> communicatively coupled to fiber optic transceivers <b>312</b>-<b>1</b> to <b>312</b>-x and optical switch <b>314</b>.

In the downlink direction, the interface functionality <b>311</b>-<b>1</b> to <b>311</b>-x receives analog downlink RF signals from the one or more base stations to which the hub <b>310</b> is communicatively coupled and provides to the fiber optical transceivers <b>312</b>-<b>1</b> to <b>312</b>-x, respectively, a suitable electrical signal for modulating onto a downlink optical carrier. The fiber optical transceivers <b>312</b>-<b>1</b> to <b>312</b>-x modulate the electrical signal onto a different downlink optical carrier. The downlink optical signal d<b>1</b>, d<b>2</b>, to dx output by each of the fiber optic transceivers <b>312</b>-<b>1</b> to <b>312</b>-x is optically multiplexed by WDM MUX <b>313</b> into a single downlink multiple wavelength optical signal (shown as d<b>1</b>+d<b>2</b>+ . . . +dx). The downlink multiple wavelength optical signal output by WDM MUX <b>313</b> is selectively communicated to the remote node <b>330</b> on either the primary downlink optical fiber <b>322</b> or secondary downlink optical fiber <b>327</b> by the optical switch <b>314</b>, depending on a fiber path control signal output by the hub optical path protection manager <b>316</b>.

In one digital-transport implementation of such an embodiment, the interface functionality <b>311</b>-<b>1</b> to <b>311</b>-x receives one or more analog downlink radio frequency (RF) signals from each of the base stations to which the hub <b>310</b> is communicatively coupled and digitizes at least a portion of the received analog downlink RF signals (for example, by digitizing a particular frequency band of each received analog downlink RF signal). Also, in such a digital-transport implementation, each item of interface functionality <b>311</b>-<b>1</b> to <b>311</b>-x combines at least a portion of the digitized downlink RF signals from one or more base stations into frames suitable for transmission on the primary downlink optical fiber <b>322</b> or the secondary downlink optical fiber <b>327</b> (for example, by formatting the at least a portion of the digitized downlink RF signals into SONET STS-48/OC-48 formatted frames). In such a digital-transport implementation, the fiber optic transceivers <b>312</b>-<b>1</b> to <b>312</b>-x each digitally modulate the electrical signal (which comprises frames of digitized downlink RF signals) onto a respective downlink optical carrier. All of the downlink optical signals output by the fiber optic transceivers <b>312</b>-<b>1</b> to <b>312</b>-x are multiplexed together by the WDM MUX <b>313</b>. An example of downlink functionality suitable for use in such a digital-transport implementation is described in U.S. Pat. No. 6,963,552, titled “MULTI-PROTOCOL DISTRIBUTED WIRELESS SYSTEM ARCHITECTURE” (also referred to here as the “'552 Patent”), which is hereby incorporated herein by reference.

In an alternative analog-transport implementation of such an embodiment, the interface functionality <b>311</b>-<b>1</b> to <b>311</b>-x each receives one or more analog downlink RF signals from each of the base stations to which the hub <b>310</b> is communicatively coupled and filters, combines, mixes, and/or splits the received analog downlink RF signals into a single electrical analog signal suitable for transmission on the primary downlink optical fiber <b>322</b> and/or the secondary downlink optical fiber <b>327</b>. In such an analog-transport implementation, the fiber optic transceivers <b>312</b>-<b>1</b> to <b>312</b>-x each amplitude modulate the single electrical analog signal received from the respective items of interface functionality <b>311</b>-<b>1</b> to <b>311</b>-x onto a respective downlink optical carrier in order to generate the respective downlink optical signal. All of the downlink optical signals output by the fiber optic transceivers <b>312</b>-<b>1</b> to <b>312</b>-x are multiplexed together by the WDM MUX <b>313</b>. In the embodiment shown in <figref idrefs="DRAWINGS">FIG. 3A</figref>, the downlink output of hub <b>310</b> is coupled to the remote nodes <b>330</b>-<b>1</b> to <b>330</b>-N via a cable vault <b>331</b> that is remotely located from the hub <b>310</b> and proximal to the first remote node <b>330</b>-<b>1</b>. The cable vault <b>331</b> comprises an optical combiner <b>334</b> that has two inputs that are coupled to the primary downlink optical fiber <b>322</b> and the secondary downlink optical fiber <b>327</b>. Optical switch <b>314</b> outputs the downlink multiple wavelength optical signal on either the primary downlink optical fiber <b>322</b> or the secondary downlink optical fiber <b>327</b>, depending on the fiber path control signal from the hub optical path protection manager <b>316</b>.

In the uplink direction, hub <b>310</b> receives an uplink multiple wavelength optical signal via one or both of primary uplink optical fiber <b>324</b> and secondary uplink optical fiber <b>329</b> from an optical splitter <b>335</b> that is also housed within the cable vault <b>331</b>. The uplink multiple wavelength optical signal comprises multiplexed uplink optical signals (shown as u<b>1</b>, u<b>2</b>, . . . ux) from the remote nodes <b>330</b>-<b>1</b> to <b>330</b>-N. Optical switch <b>314</b> selectively couples one of the primary uplink optical fiber <b>324</b> and the secondary uplink optical fiber <b>329</b> to the WDM DE-MUX <b>315</b>, depending on the fiber path control signal from the hub optical path protection manager <b>316</b>. That is, when optical switch <b>314</b> selectively couples the primary uplink optical fiber <b>324</b> to the WDM DE-MUX <b>315</b>, any uplink multiple wavelength optical signal received on<b>1</b> the primary uplink optical fiber <b>324</b> is communicated to the WDM DE-MUX <b>315</b> by the optical switch <b>314</b>. Likewise, when optical switch <b>314</b> selectively couples the secondary uplink optical fiber <b>329</b> to the WDM DE-MUX <b>315</b>, any uplink multiple wavelength optical signal received on the secondary uplink optical fiber <b>329</b> is communicated to the WDM DE-MUX <b>315</b> by the optical switch <b>314</b>. WDM DE-MUX <b>315</b> de-multiplexes the uplink multiple wavelength optical signal into x single wavelength optical signals, and forwards each single wavelength optical signal to one of the fiber optic transceivers <b>312</b>-<b>1</b> to <b>312</b>-x. Each of the fiber optic transceivers <b>312</b>-<b>1</b> to <b>312</b>-x demodulates the uplink optical signal they receive from WDM DE-MUX <b>315</b> in order to extract an electrical uplink RF signal, which is respectively provided to one or more base stations via the interface functionality <b>311</b>-<b>1</b> to <b>311</b>-x. In the digital-transport implementation of such an embodiment noted above, the extracted uplink RF signal comprises frames (for example, SONET STS-48/OC-48 formatted frames) containing digitized uplink RF data, which the interface functionality <b>311</b>-<b>1</b> to <b>311</b>-x extracts from the frames and converts to analog uplink RF signals. The analog uplink RF signals, in such an implementation, are provided to one or more base stations coupled to the hub <b>310</b>. In the analog-transport implementation of such an embodiment noted above, the extracted uplink RF signal comprises analog uplink RF signals that are provided to one or more base stations coupled to the hub <b>310</b> (for example, with appropriate amplification and filtering). In the embodiment shown in <figref idrefs="DRAWINGS">FIG. 3A</figref>, optical combiner <b>334</b> operates as described above with respect to optical combiners <b>134</b> and <b>235</b> to combine any downlink multiple wavelength optical signal received oil primary downlink optical fiber <b>322</b> and secondary downlink optical fiber <b>327</b> into a single downlink multiple wavelength optical output. Optical splitter <b>335</b> operates as described above with respect to optical splitters <b>135</b> and <b>235</b> to replicate any uplink multiple wavelength optical signal for communication to hub <b>310</b> via both of the primary uplink optical fiber <b>324</b> and the secondary uplink optical fiber <b>329</b>.

In the embodiment shown in <figref idrefs="DRAWINGS">FIG. 3A</figref>, remote nodes <b>330</b>-<b>1</b> to <b>330</b>-N are communicatively coupled to one another in a “daisy chain” topology. Each of the remote nodes <b>330</b>-<b>1</b> to <b>330</b>-N is coupled to optical combiner <b>334</b> and optical splitter <b>335</b> through add/drop multiplexers <b>350</b>-<b>1</b> to <b>350</b>-M.

In the downlink direction, a first add/drop multiplexer <b>350</b>-<b>1</b> is coupled to optical combiner <b>334</b> and receives the downlink multiple wavelength signal that comprises a multiplexed version of the RF data signal outputs from fiber optic transceivers <b>312</b>-<b>1</b> to <b>312</b>-x (shown as d<b>1</b>+d<b>2</b>+ . . . +dx). Add/drop multiplexer <b>350</b>-<b>1</b> “drops” downlink optical signal d<b>1</b> (which is the downlink optical signal output by the fiber optic transceiver <b>330</b>-<b>1</b>) to first remote node <b>330</b>-<b>1</b> over a downlink optical fiber. In the implementation shown in <figref idrefs="DRAWINGS">FIG. 3A</figref>, Add/drop multiplexer <b>350</b>-<b>1</b> also outputs the received downlink multiple wavelength signal minus the downlink optical signal d<b>1</b> dropped to the first remote node <b>330</b>-<b>1</b> (shown as d<b>2</b>+ . . . +dx) to the next add/drop multiplexer in the daisy chain. The next add/drop multiplexer in the downlink direction receives the signal from the first add/drop multiplexer, similarly drops the second downlink optical signal d<b>2</b> to the second remote node <b>330</b>-<b>2</b>, and outputs the remaining signal to next add/drop multiplexer in the daisy chain. Each of the add/drop multiplexers in the daisy chain similarly drop the respective downlink optical signal to the respective remote unit until the last remote node <b>330</b>-N receives the last downlink optical signal (shown as dx). In alternate implementations, the downlink optical signals are not necessarily removed from the fiber as they are dropped to their associated remote nodes. For example, in one such alternate implementation, Add/drop multiplexer <b>350</b>-<b>1</b> outputs the same downlink multiple wavelength signal it received to the next add/drop multiplexer in the daisy chain.

In the uplink direction each of the add/drop multiplexers <b>350</b>-<b>1</b> to <b>350</b>-M adds a respective uplink optical signal received from a respective remote node to the previously multiplexed optical signals from the daisy chain. For example, add/drop multiplexer <b>350</b>-M multiplexes together uplink optical signals received from the last remote node <b>330</b>-N and the second to last remote node <b>330</b>-(N-<b>1</b>) to produce an uplink multiple wavelength optical signal (illustrated in <figref idrefs="DRAWINGS">FIG. 3A</figref> as u(x−<b>1</b>)+ux). The next upstream add/drop multiplexer receives that uplink multiple wavelength optical signal u(x−<b>1</b>)+ux and adds it together with an uplink optical signal received from its associated remote node. Thus the uplink multiple wavelength optical signal received by optical splitter <b>335</b> comprises a multiplexed version of the uplink multiple wavelength optical signal (shown as u<b>1</b>+u<b>2</b>+ . . . +ux).

Each of the remote nodes <b>330</b>-<b>1</b> to <b>330</b>-N comprise a fiber optic transceiver, interface functionality, and a remote optical path protection (OPP) manager that function as described with respect to remote node <b>230</b> illustrated in <figref idrefs="DRAWINGS">FIG. 2</figref>. For example, remote node <b>330</b>-<b>1</b>, evaluates the downlink signal quality of the downlink optical signal received from the hub <b>310</b>. In one implementation, the remote nodes <b>330</b>-<b>1</b>-<b>330</b>-N determines the bit error rate (BER) of the respective downlink optical signal received at that remote node. When the BER measured by from one or more of the remote nodes <b>330</b>-<b>1</b>-<b>330</b>-N drops below a predetermined threshold level, that remote node reports the BER to hub <b>310</b> by any of the means described with respect to <figref idrefs="DRAWINGS">FIGS. 1 and 2</figref>. Hub <b>310</b> can then make the determination on whether to realign optical switch <b>314</b> from the primary fiber path <b>320</b> to the backup fiber path <b>325</b>. In other implementations, other signal quality indicators are used to determine whether or not communications via primary fiber path <b>320</b> are within acceptable operating parameters. For example, in another implementation each of the remote nodes <b>330</b>-<b>1</b>-<b>330</b>-N determine the optical power level of the downlink optical signal it receives. In such an implementation, when the optical power level drops below a predetermined threshold power level, the remote nodes reports the optical power level to hub <b>310</b> by any of the means described with respect to <figref idrefs="DRAWINGS">FIGS. 1 and 2</figref>. Hub <b>310</b> can then make the determination on whether to realign optical switch <b>314</b> from the primary fiber path <b>320</b> to the backup fiber path <b>325</b>.

In one implementation, hub <b>310</b> determines whether to switch from primary fiber path <b>320</b> to the backup fiber path <b>325</b> based on all of the signal quality feedback provided for the remote nodes <b>330</b>-<b>1</b> to <b>330</b>-N. For example, in one implementation, if a downlink BER or optical power level reported by any one of the remote nodes <b>330</b>-<b>1</b> to <b>330</b>-N indicates degrading optical signal quality, but downlink BERs or optical power levels reported by the other remote nodes do not, the hub <b>310</b> concludes that the degrading optical signal quality is due to a local problem with the one remote node, rather than a degradation of the primary fiber path <b>320</b>. However, when all of the remote nodes <b>330</b>-<b>1</b> to <b>330</b>-N report degrading downlink optical signal quality, then hub <b>310</b> concludes that the degrading optical signal quality is due to a degradation of the primary fiber path <b>320</b> and switches to the backup fiber path <b>325</b>. Similarly, if hub <b>310</b> detects the loss of an uplink optical signal from one of the remote nodes <b>330</b>-<b>1</b> to <b>330</b>-N, but continues to detect uplink optical signals from the other remote nodes, the hub <b>310</b> concludes that the degrading optical signal quality is due to a local problem with that remote node, rather than a degradation of the primary fiber path <b>320</b>. However, when hub <b>310</b> detects a loss of uplink optical signals from all of the remote nodes <b>330</b>-<b>1</b> to <b>330</b>-N, then hub <b>310</b> concludes that the degrading optical signal quality is due to a degradation of the primary fiber path <b>320</b> and switches to the backup fiber path <b>325</b>.

Although the embodiments illustrated in this specification describe optical path protection systems and methods in terms of switching from a designated primary fiber path to a designated backup fiber path upon detection of primary fiber path degradation, one of ordinary skill in the art upon reading this specification would appreciate that the same systems and methods are applicable for switching from the designated backup fiber path back to the designated primary fiber path. Further, the designation in the field of which fiber path is the primary fiber path and which is the backup fiber path is arbitrarily determined by the system operator. As such embodiments of the present invention include embodiments for switching between any first fiber path and any second fiber path. In some embodiments, hysteresis is including in the switching determination made by a hub to prevent continual switching (also called ‘Flapping’) between two fiber paths that are both degraded but not failed.

<figref idrefs="DRAWINGS">FIG. 3A</figref> illustrates and embodiment where each of the remote units <b>330</b>-<b>1</b> to <b>330</b>-N is associated with its own add/drop multiplexer. <figref idrefs="DRAWINGS">FIG. 3B</figref> illustrates one embodiment where a single mux/demux <b>355</b> is located at a convenient installation near remote units <b>330</b>-<b>1</b> to <b>330</b>-N. Instead of daisy chaining the multiple add/drop multiplexers <b>350</b>-<b>1</b> to <b>350</b>-M, to communicate the uplink optical signals (u<b>1</b> to ux) and downlink optical signals (d<b>1</b> to dx) with remote units <b>330</b>-<b>1</b> to <b>330</b>-N, in the embodiment of <figref idrefs="DRAWINGS">FIG. 3B</figref>, mux/demux <b>355</b> is directly coupled to each of remote units <b>330</b>-<b>1</b> to <b>330</b>-N, optical combiner <b>334</b> and optical splitter <b>335</b>. Mux/demux <b>355</b> includes the functionality required to receive the downlink multiple wavelength signal (shown as d<b>1</b>+d<b>2</b>+ . . . +dx) from optical combiner <b>334</b> and distribute each of the downlink optical signals (d<b>1</b> to dx) within the downlink multiple wavelength signal to their corresponding remote unit of remote units <b>330</b>-<b>1</b> to <b>330</b>-N. Mux/demux <b>355</b> includes the functionality required to receive the uplink optical signals (u<b>1</b> to ux) from remote units <b>330</b>-<b>1</b> to <b>330</b>-N, multiplex the uplink optical signals to into the uplink multiple wavelength signal (shown as u<b>1</b>+u<b>2</b>+ . . . +ux), and communicate the uplink multiple wavelength signal to optical splitter <b>335</b>.

<figref idrefs="DRAWINGS">FIG. 4</figref> is a flow chart illustrating a method for providing optical path protection for a distributed antenna system. In one implementation, the method is performed by a distributed antenna system as described with respect to <figref idrefs="DRAWINGS">FIGS. 1</figref>, <b>2</b> and <b>3</b>. The method begins at <b>410</b> with transmitting a downlink optical signal to a downlink fiber of one of a first fiber path and a second fiber path. In one implementation, the first fiber path functions as a primary fiber path while the second fiber path functions as a backup fiber path. In other implementations, the first fiber path functions as backup fiber path while the second fiber path functions as the primary fiber path. In one implementation, transmitting the downlink optical signal is performed from a hub of the distributed antenna system. The method continues at <b>415</b> with combining any downlink optical signal received on the downlink fiber of the first fiber path and any downlink optical signal received on the downlink fiber of the second fiber path in order to output the downlink optical signal. Combining any downlink optical signals from the first and second fiber paths at <b>415</b> while transmitting a downlink optical signal at <b>410</b> to only one of those paths enables the passive reception of the downlink optical signal without a-priori knowledge of whether the downlink optical signal is transmitted on the first or second fiber path. The method continues at <b>420</b> with splitting an uplink optical signal for transmission on an uplink fiber of the first fiber path and an uplink fiber of the second fiber path. Splitting the uplink optical signal for transmission on both the first and second optical fiber paths enables the passive transmission of the uplink optical signal from a remote node without the need for a-priori knowledge of whether a hub is aligned to receive uplink optical signals from the first or second optical fiber path. In one implementation, transmitting the uplink optical signal is performed from a remote node of the distributed antenna system. The method proceeds to <b>430</b> with determining a signal quality of the downlink optical signal. In one implementation of the method, determining the signal quality of the downlink optical signal comprises calculating the bit error rate of the downlink optical signal. In one implementation of the method, determining the signal quality of the downlink optical signal comprises determining the optical power level of the downlink optical signal. In one implementation, when the downlink optical signal comprises multiple wavelengths, determining the signal quality of the downlink optical signal comprises determining the bit error rate for data communicated in one of the multiple wavelengths. In one implementation, a remote node communicates the signal quality of the downlink optical signal back to the hub. Communication of the signal quality to the hub can include transmitting signal quality data to the hub via an uplink optical signal, or alerting the hub of inadequate downlink optical signal quality by disabling the uplink optical signal. The method proceeds to <b>440</b> with determining a signal quality of the uplink optical signal. In one implementation of the method, determining the signal quality of the uplink optical signal comprises calculating the bit error rate of the uplink optical signal. In one implementation, when the uplink optical signal comprises multiple wavelengths, determining the signal quality of the downlink optical signal comprises determining the bit error rate for data communicated in one of the multiple wavelengths. The method proceeds to <b>450</b> with switching between a normal state and backup state based on one or both of the signal quality of the uplink optical signal and the signal quality of the downlink optical signal. In one implementation of the method when switched to a normal state, downlink optical signals are transmitted on the downlink fiber of the first fiber path and uplink optical signals are received on the uplink fiber of the first fiber path. In such an implementation, when switched to the backup state, downlink optical signals are transmitted on the downlink fiber of the second fiber path and uplink optical signals are received on the uplink fiber of the second fiber path. In another implementation of the method, when switched to the normal state, downlink optical signals are transmitted on the downlink fiber of the second fiber path and uplink optical signals are received on the uplink fiber of the second fiber path. In such an implementation of the method, when switched to the backup state, downlink optical signals are transmitted on the downlink fiber of the first fiber path and uplink optical signals are received on the uplink fiber of the first fiber path.

In one implementation, when a hub is switched to utilize the first fiber path and either the signal quality of the downlink optical signal (determined at <b>430</b>) or the signal quality of the uplink optical signal (determined at <b>440</b>) is inadequate, the hub switches from the first fiber path to the second fiber bath. Conversely, when the hub is switched to utilize the second fiber path and either the signal quality of the downlink optical signal (determined at <b>430</b>) or the signal quality of the uplink optical signal (determined at <b>440</b>) is inadequate, the hub switches from the second fiber path to the first fiber path. In one implementation of the method, hysteresis is including in the switching determination made by a hub to prevent continual switching between the first and second fiber paths when both are degraded but not failed.

As would be appreciated by one skilled in the art upon reading this specification, a single fiber can be used to communicate both an uplink optical signal and a downlink optical signal simultaneously by appropriate allocation of wavelengths. Such implementations are included within the scope of embodiments of the present invention, as illustrated by the embodiment of <figref idrefs="DRAWINGS">FIG. 5</figref>. In the embodiment of <figref idrefs="DRAWINGS">FIG. 5</figref>, hub <b>510</b> communicates with a plurality of remote nodes <b>530</b>-<b>1</b> to <b>530</b>-N via multiple wavelength optical signals. Hub <b>510</b> includes the functionality required to multiplex multiple optical carrier signals from a communications network (not shown) onto a single optical fiber by using different wavelengths of light to carry a plurality of different RF signals. Hub <b>510</b> comprises interface functionality <b>511</b>-<b>1</b> to <b>511</b>-x that couple the hub <b>310</b> to one or more base stations (not shown). The hub <b>510</b> further comprises fiber optic transceivers <b>512</b>-<b>1</b> to <b>512</b>-x communicatively coupled, respectively, to the interface functionality <b>511</b>-<b>1</b> to <b>511</b>-x. The hub <b>510</b> further comprises a hub optical path protection manager (HOM) <b>516</b> communicatively coupled to fiber optic transceivers <b>512</b>-<b>1</b> to <b>512</b>-x and optical switch <b>514</b>.

Hub <b>510</b> further comprises a wave division multiplexing (WDM) multiplexer/demultiplexer (MUX/DE-MUX) <b>513</b> that is communicatively coupled to the downlink output of fiber optical transceivers <b>312</b>-<b>1</b> to <b>312</b>-x and the uplink inputs of optical transceivers <b>312</b>-<b>1</b> to <b>312</b>-x, and further comprises an optical switch <b>514</b> communicatively coupled to WDM MUX/DE-MUX <b>513</b>, primary optical fiber path <b>520</b> and backup optical fiber path <b>525</b>. In the embodiment of <figref idrefs="DRAWINGS">FIG. 5</figref>, primary optical fiber path <b>520</b> comprises a common fiber used to communicate both an uplink multiple wavelength signal (shown as u<b>1</b>+u<b>2</b>+ . . . +ux) from hub <b>510</b> to remote nodes <b>530</b>-<b>1</b> to <b>530</b>-N and a downlink multiple wavelength signal (shown as d<b>1</b>+d<b>2</b>+ . . . +dx) from the remote nodes <b>530</b>-<b>1</b> to <b>530</b>-N to hub <b>510</b>. Backup optical fiber path <b>525</b> comprises another common fiber used to communicate both an uplink multiple wavelength signal from hub <b>510</b> to remote nodes <b>530</b>-<b>1</b> to <b>530</b>-N and a downlink multiple wavelength signal from the remote nodes <b>530</b>-<b>1</b> to <b>530</b>-N to hub <b>510</b>.

In the downlink direction, the interface functionality <b>511</b>-<b>1</b> to <b>511</b>-x receives analog downlink RF signals from the one or more base stations to which the hub <b>510</b> is communicatively coupled and provides to the fiber optical transceivers <b>512</b>-<b>1</b> to <b>512</b>-x, respectively, a suitable electrical signal for modulating onto a downlink optical carrier. The fiber optical transceivers <b>512</b>-<b>1</b> to <b>512</b>-x modulate the electrical signal onto a different downlink optical carrier. The downlink optical signal (d<b>1</b>, d<b>2</b>, to dx) output by each of the fiber optic transceivers <b>512</b>-<b>1</b> to <b>512</b>-x is optically multiplexed by WDM MUX/DE-MUX <b>513</b> into the single downlink multiple wavelength optical signal (shown as d<b>1</b>+d<b>2</b>+ . . . +dx). The downlink multiple wavelength optical signal output by WDM MUX/DE-MUX <b>513</b> is selectively communicated to the remote node <b>330</b> on either the primary optical fiber path <b>520</b> or the backup optical fiber path <b>525</b> by the optical switch <b>514</b>, depending on a fiber path control signal output by the hub optical path protection manager <b>516</b>.

In one digital-transport implementation of such an embodiment, the interface functionality <b>511</b>-<b>1</b> to <b>511</b>-x receives one or more analog downlink radio frequency (RF) signals from each of the base stations to which the hub <b>510</b> is communicatively coupled and digitizes at least a portion of the received analog downlink RF signals (for example, by digitizing a particular frequency band of each received analog downlink RF signal). Also, in such a digital-transport implementation, each item of interface functionality <b>511</b>-<b>1</b> to <b>511</b>-<i>x </i>combines at least a portion of the digitized downlink RF signals from one or more base stations into frames suitable for transmission on the primary optical fiber path <b>520</b> or the backup optical fiber path <b>525</b> (for example, by formatting the at least a portion of the digitized downlink RF signals into SONET STS-48/OC-48 formatted frames). In such a digital-transport implementation, the fiber optic transceivers <b>512</b>-<b>1</b> to <b>512</b>-x each digitally modulate the electrical signal (which comprises frames of digitized downlink RF signals) onto a respective downlink optical carrier. All of the downlink optical signals output by the fiber optic transceivers <b>512</b>-<b>1</b> to <b>512</b>-x are multiplexed together by the WDM MUX/DE-MUX <b>513</b>. An example of downlink functionality suitable for use in such a digital-transport implementation is described in U.S. Pat. No. 6,963,552, titled “MULTI-PROTOCOL DISTRIBUTED WIRELESS SYSTEM ARCHITECTURE” (also referred to here as the “'552 Patent”), which is hereby incorporated herein by reference.

In an alternative analog-transport implementation of such an embodiment, the interface functionality <b>511</b>-<b>1</b> to <b>511</b>-x each receives one or more analog downlink RF signals from each of the base stations to which the hub <b>510</b> is communicatively coupled and filters, combines, mixes, and/or splits the received analog downlink RF signals into a single electrical analog signal suitable for transmission on the primary optical fiber path <b>520</b> or the backup optical fiber path <b>525</b>. In such an analog-transport implementation, the fiber optic transceivers <b>512</b>-<b>1</b> to <b>512</b>-x each amplitude modulate the single electrical analog signal received from the respective items of interface functionality <b>511</b>-<b>1</b> to <b>511</b>-x onto a respective downlink optical carrier in order to generate the respective downlink optical signal. All of the downlink optical signals output by the fiber optic transceivers <b>512</b>-<b>1</b> to <b>512</b>-x are multiplexed together by the WDM MUX/DE-MUX <b>513</b>. In the embodiment shown in <figref idrefs="DRAWINGS">FIG. 5</figref>, the downlink output of hub <b>510</b> is coupled to the remote nodes <b>530</b>-<b>1</b> to <b>530</b>-N via a cable vault <b>531</b> that is remotely located from the hub <b>510</b> and proximal to the first remote node <b>530</b>-<b>1</b>. The cable vault <b>531</b> comprises an optical combiner/splitter <b>534</b> that has two inputs that are coupled to the primary optical fiber path <b>520</b> and the backup optical fiber path <b>525</b>. Optical switch <b>514</b> outputs the downlink multiple wavelength optical signal on either the primary optical fiber path <b>520</b> or the backup optical fiber path <b>525</b>, depending on the fiber path control signal from the hub optical path protection manager <b>516</b>.

In the uplink direction, hub <b>510</b> receives an uplink multiple wavelength optical signal via one or both of primary optical fiber path <b>520</b> and the backup optical fiber path <b>525</b> from optical combiner/splitter <b>534</b>. The uplink multiple wavelength optical signal comprises multiplexed uplink optical signals (shown as u<b>1</b>, u<b>2</b>, . . . ux) from the remote nodes <b>530</b>-<b>1</b> to <b>530</b>-N. Optical switch <b>514</b> selectively couples one of the primary optical fiber path <b>520</b> or the backup optical fiber path <b>525</b> to the WDM MUX/DE-MUX <b>513</b>, depending on the fiber path control signal from the hub optical path protection manager <b>516</b>. That is, when optical switch <b>514</b> selectively couples the primary optical fiber path <b>520</b> to the WDM MUX/DE-MUX <b>513</b>, any uplink multiple wavelength optical signal received on the primary optical path <b>520</b> is communicated to the WDM MUX/DE-MUX <b>513</b> by the optical switch <b>514</b>. Likewise, when optical switch <b>514</b> selectively couples the backup optical path <b>525</b> to the WDM MUX/DE-MUX <b>513</b>, any uplink multiple wavelength optical signal received on the backup optical path <b>525</b> is communicated to the WDM MUX/DE-MUX <b>513</b> by the optical switch <b>514</b>.

WDM MUX/DE-MUX <b>513</b> de-multiplexes the uplink multiple wavelength optical signal into x single wavelength optical signals, and forwards each single wavelength optical signal to one of the fiber optic transceivers <b>512</b>-<b>1</b> to <b>512</b>-x. Each of the fiber optic transceivers <b>512</b>-<b>1</b> to <b>512</b>-x demodulates the uplink optical signal they receive from WDM MUX/DE-MUX <b>315</b> in order to extract an electrical uplink RF signal, which is respectively provided to one or more base stations via the interface functionality <b>511</b>-<b>1</b> to <b>511</b>-x. In the digital-transport implementation of such an embodiment noted above, the extracted uplink RF signal comprises frames (for example, SONET STS-48/OC-48 formatted frames) containing digitized uplink RF data, which the interface functionality <b>511</b>-<b>1</b> to <b>511</b>-x extracts from the frames and converts to analog uplink RF signals. The analog uplink RF signals, in such an implementation, are provided to one or more base stations coupled to the hub <b>510</b>. In the analog-transport implementation of such an embodiment noted above, the extracted uplink RF signal comprises analog uplink RF signals that are provided to one or more base stations coupled to the hub <b>510</b> (for example, with appropriate amplification and filtering). In the embodiment shown in <figref idrefs="DRAWINGS">FIG. 5</figref>, optical combiner/splitter <b>534</b> operates in the downlink direction as described above with respect to optical combiners <b>134</b> and <b>235</b> to combine any downlink multiple wavelength optical signal received on primary optical path <b>520</b> and backup optical path <b>525</b> into a single downlink multiple wavelength optical output. Optical combiner/splitter <b>535</b> operates in the uplink direction as described above with respect to optical splitters <b>135</b> and <b>235</b> to replicate any uplink multiple wavelength optical signal for communication to hub <b>510</b> via both of the primary optical path <b>520</b> and the backup optical path <b>525</b>.

In the embodiment shown in <figref idrefs="DRAWINGS">FIG. 5</figref>, remote nodes <b>530</b>-<b>1</b> to <b>530</b>-N are communicatively coupled to one another in a “daisy chain” topology. Each of the remote nodes <b>530</b>-<b>1</b> to <b>530</b>-N is coupled to optical combiner/splitter <b>534</b> through add/drop multiplexers <b>550</b>-<b>1</b> to <b>550</b>-M. In the embodiment shown in <figref idrefs="DRAWINGS">FIG. 5</figref>, each of the add/drop multiplexers <b>550</b>-<b>1</b> to <b>550</b>-M are coupled to their adjacent add/drop multiplexer using a common used to communicate both the uplink optical signals and downlink optical signals simultaneously by appropriate allocation of wavelengths. The first add/drop multiplexer <b>550</b>-<b>1</b> is coupled to optical combiner/splitter <b>534</b> via a common fiber used to receive the downlink multiple wavelength signal from the optical combiner/splitter <b>534</b> and to send the uplink downlink multiple wavelength signal to the optical combiner/splitter <b>534</b>.

Add/drop multiplexers <b>550</b>-<b>1</b> to <b>550</b>-M each include the functionality described with respect to add/drop multiplexers <b>350</b>-<b>1</b> to <b>350</b>-M. In the downlink direction first add/drop multiplexer <b>550</b>-<b>1</b> receives the downlink multiple wavelength signal (shown as d<b>1</b>+d<b>2</b>+ . . . +dx), and “drops” downlink optical signal d<b>1</b> to first remote node <b>530</b>-<b>1</b> over an optical fiber. In the implementation shown in <figref idrefs="DRAWINGS">FIG. 5</figref>, add/drop multiplexer <b>550</b>-<b>1</b> also outputs the received downlink multiple wavelength signal minus the downlink optical signal d<b>1</b> dropped to the first remote node <b>530</b>-<b>1</b> (shown as d<b>2</b>+ . . . +dx) to the next add/drop multiplexer in the daisy chain. The next add/drop multiplexer in the downlink direction receives the signal from the first add/drop multiplexer, similarly drops the second downlink optical signal d<b>2</b> to the second remote node <b>530</b>-<b>2</b>, and outputs the remaining signal to next add/drop multiplexer in the daisy chain. Each of the add/drop multiplexers in the daisy chain similarly drop the respective downlink optical signal to the respective remote unit until the last remote node <b>530</b>-N receives the last downlink optical signal (shown as dx). In alternate implementations, the downlink optical signals are not necessarily removed from the fiber as they are dropped to their associated remote nodes. For example, in one such alternate implementation, Add/drop multiplexer <b>550</b>-<b>1</b> outputs the same downlink multiple wavelength signal it received to the next add/drop multiplexer in the daisy chain.

In the uplink direction each of the add/drop multiplexers <b>550</b>-<b>1</b> to <b>550</b>-M adds a respective uplink optical signal received from a respective remote node to the previously multiplexed optical signals from the daisy chain. In one implementation an add/drop multiplexer receives an uplink optical signal from their associated remote node over the same optical fiber used to send the downlink optical signal to the remote node. For example, add/drop multiplexer <b>550</b>-M multiplexes together uplink optical signals received from the last remote node <b>530</b>-N and the second to last remote node <b>530</b>-(N-<b>1</b>) to produce an uplink multiple wavelength optical signal (illustrated in <figref idrefs="DRAWINGS">FIG. 5</figref> as u(x−<b>1</b>)+ux). The next upstream add/drop multiplexer receives that uplink multiple wavelength optical signal u(x−<b>1</b>)+ux and adds it together with an uplink optical signal received from its associated remote node. Thus the uplink multiple wavelength optical signal received by optical splitter <b>535</b> comprises a multiplexed version of the uplink multiple wavelength optical signal (shown as u<b>1</b>+u<b>2</b>+ . . . +ux).

Each of the remote nodes <b>530</b>-<b>1</b> to <b>530</b>-N comprise a fiber optic transceiver, interface functionality, and a remote optical path protection (OPP) manager that function as described with respect to remote node <b>230</b> illustrated in <figref idrefs="DRAWINGS">FIG. 2</figref>. For example, remote node <b>530</b>-<b>1</b>, evaluates the downlink signal quality of the downlink optical signal received from the hub <b>510</b>. In one implementation, the remote nodes <b>530</b>-<b>1</b>-<b>530</b>-N determines the bit error rate (BER) of the respective downlink optical signal received at that remote node. When the BER measured by from one or more of the remote nodes <b>530</b>-<b>1</b>-<b>530</b>-N drops below a predetermined threshold level, that remote node reports the BER to hub <b>510</b> by any of the means described with respect to <figref idrefs="DRAWINGS">FIGS. 1 and 2</figref>. Hub <b>510</b> can then make the determination on whether to realign optical switch <b>514</b> from the primary fiber path <b>520</b> to the backup fiber path <b>525</b>. In other implementations, other signal quality indicators are used to determine whether or not communications via primary fiber path <b>520</b> are within acceptable operating parameters. For example, in another implementation each of the remote nodes <b>530</b>-<b>1</b>-<b>530</b>-N determine the optical power level of the downlink optical signal it receives. In such an implementation, when the optical power level drops below a predetermined threshold power level, the remote nodes reports the optical power level to hub <b>510</b> by any of the means described with respect to <figref idrefs="DRAWINGS">FIGS. 1 and 2</figref>. Hub <b>510</b> can then make the determination on whether to realign optical switch <b>514</b> from the primary fiber path <b>520</b> to the backup fiber path <b>525</b>.

In one implementation, hub <b>510</b> determines whether to switch from primary fiber path <b>520</b> to the backup fiber path <b>525</b> based on all of the signal quality feedback provided for the remote nodes <b>530</b>-<b>1</b> to <b>530</b>-N. For example, in one implementation, if a downlink BER or optical power level reported by any one of the remote nodes <b>530</b>-<b>1</b> to <b>530</b>-N indicates degrading optical signal quality, but downlink BERs or optical power levels reported by the other remote nodes do not, the hub <b>510</b> concludes that the degrading optical signal quality is due to a local problem with the one remote node, rather than a degradation of the primary fiber path <b>520</b>. However, when all of the remote nodes <b>530</b>-<b>1</b> to <b>530</b>-N report degrading downlink optical signal quality, then hub <b>510</b> concludes that the degrading optical signal quality is due to a degradation of the primary fiber path <b>520</b> and switches to the backup fiber path <b>525</b>. Similarly, if hub <b>510</b> detects the loss of an uplink optical signal from one of the remote nodes <b>530</b>-<b>1</b> to <b>530</b>-N, but continues to detect uplink optical signals from the other remote nodes, the hub <b>510</b> concludes that the degrading optical signal quality is due to a local problem with that remote node, rather than a degradation of the primary fiber path <b>520</b>. However, when hub <b>510</b> detects a loss of uplink optical signals from all of the remote nodes <b>530</b>-<b>1</b> to <b>530</b>-N, then hub <b>510</b> concludes that the degrading optical signal quality is due to a degradation of the primary fiber path <b>520</b> and switches to the backup fiber path <b>525</b>.

It would be appreciated by one skilled in the art upon reading this specification that the embodiments of the present invention are not limited to the transport of analog RF signals but includes the transport of analog RF signals and digital RF signals, or any combination thereof in either the uplink or downlink directions. For example, in one implementation of a network such as those describe above, one or more of a hub's interface functionality communicate wirelessly to one or more base stations via digital RF signals while one or more of the remote nodes communicate wirelessly via analog RF signals. Such an implementation may include, but is not limited to a Universal Mobile Telecommunications System/Wideband Code Division Multiple Access (UMTS/WCDMA) network.